1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device, such as a thin film transistor (hereinafter referred to as a TFT), using a crystalline semiconductor film formed on a substrate. The semiconductor device fabricated by the present invention includes not only an element such as a TFT or a MOS transistor, but also a liquid crystal display device, an EL (Electro Luminescence) display device, an EC (Electro Chromic) display device, an image sensor, or the like, each including a semiconductor circuit (microprocessor, signal processing circuit, high frequency circuit, etc.) constituted by insulated gate transistors.
2. Description of the Related Art
At present, as a semiconductor element using a semiconductor film, a thin film transistor (hereinafter referred to as a TFT) is used for each integrated circuit, and is especially used as a switching element of an image display device. Further, a TFT using as an active layer a crystalline semiconductor film having higher mobility than an amorphous semiconductor film has high drive capability and is also used as an element of a driving circuit.
As a method of obtaining a crystalline semiconductor film, a thermal annealing method, a laser annealing method, or techniques disclosed in Japanese Patent Application Laid-Open No. Hei. 7-130652 and No. Hei. 8-78329 by the present assignee are well known. The technique disclosed in the publications uses a metal element (especially nickel (Ni)) for promoting crystallization of silicon to enable the formation of a crystalline silicon film having excellent crystallinity by a heat treatment at 500 to 600° C. for about 4 hours.
Reliability is regarded as most important in evaluation of a TFT. As a factor to lower the reliability, an impurity in the TFT (hereinafter, an impurity to lower the reliability of a TFT is called a contaminant impurity in the present specification) can be mentioned. The contaminant impurity is mixed in the TFT from various contaminating sources such as the air, a glass substrate, and a manufacturing apparatus. Particularly, that the contaminant impurity exists on an interface of films constituting the TFT is a great factor to damage the reliability of the TFT.
Besides, among things indicating electrical characteristics of a TFT, there is a threshold voltage (Vth). In general, there is a method of adding boron (B) as an impurity element to give a p-type into a crystalline semiconductor film in order to control the threshold voltage (Vth) of an n-channel TFT.
It is difficult to continuously conduct all steps of forming and treating respective films constituting a TFT. For example, a crystalline semiconductor film is obtained by using a method of crystallizing an amorphous semiconductor film by heat, laser light or the like. In general, a film formation apparatus and a furnace or a laser apparatus, for a heat treatment are respectively independent of each other. Besides, if a film formation apparatus is used, in which only one kind of film can be formed, though the apparatus includes a single or a plurality of film formation chambers, the surface of a substrate is exposed to a clean room atmosphere each time the film formation is made. Accordingly, it becomes necessary to provide a step of washing the surface of a film prior to a next treatment. However, the substrate is also exposed to the air atmosphere until it is put in the film formation apparatus from a washer.
A semiconductor device is generally fabricated in a clean room. In the clean room, although a filter is used to remove dirt, dust, contaminant substance or the like from the outer air to be taken in, a lot of contaminant impurities generated from the filter itself, especially boron (B) or a lot of contaminant impurities from a person working in the clean room, especially sodium (Na) exist in the clean room atmosphere. That is, the surface of the substrate is contaminated by merely exposing the substrate to the clean room atmosphere.
Besides, if an impurity is put in a regular crystal structure, the regular crystal structure is broken. That is, that the boron (B) to give a p-type is added to a crystalline semiconductor film in order to control a threshold voltage (Vth) breaks the crystal structure of the crystalline semiconductor film, and there is a possibility that the drive capability of the TFT is lowered.